In a Phoenix Framework form, how can I set a belongs_to relation back to null using a changeset?

Question

In a Phoenix Framework form, I have a select box on my page which has an option to set a belongs_to value to nil.

<%= select f, :relation_id, 
  Enum.into(Enum.map(@relations, fn p -> {p.name, p.id} end), 
  [{"None", nil}]) %>

The form would usually send the ID, but when the nil value is selected, it passes the value as an empty string:

"relation_id" => ""

I receive an error from Ecto that the changeset is invalid, as it expects an integer. I could probably intercept the map, set the value to null, and pass the updated map into the changeset. But is there an easier way to do this?

How do you preload the relations field in your controller? Can't find an example on how to do it with a changeset. Thanks — Paco
Ahh - the select just expects a basic list of tuples and can be generated however you like. In this case, @relations was just a straight pull using Repo.all: relations = Repo.all(relations_query), and then passed into the view in the render command. — Don Pflaster

Fabi755 Fabi755 · Accepted Answer · 2016-02-03T16:44:35

I think you should use the plug scrub params.

Try to add to your controller:

defmodule MyApp.SomeThingController do
  use MyApp.Web, :controller

  plug :scrub_params, "some_thing" when action in [:create, :update]

  # def ....
end

It will be convert "" (empty) values to nil values.

Hope it helps.

In a Phoenix Framework form, how can I set a belongs_to relation back to null using a changeset?

1 Answers